1. Technical Field
The present invention relates generally to solenoids and circuits for monitoring their operation. More particularly, the invention relates to a system for monitoring the solenoid flyback voltage spike which can be implemented within a vehicle based controller.
2. Discussion
Solenoids are electromechanical force actuating devices that may be combined with a computer system to provide control over hydraulic or mechanical equipment. Electrical signals from the computer instruct the solenoids to open or close a valve or mechanical linkage providing fine-tuned control over the operation of the equipment. With the continued decrease in the cost of microprocessors, the use of solenoids to control equipment has increased. Some examples of solenoid controlled equipment include: automotive transmissions, fuel injectors, A/C systems, aircraft control systems, and industrial manufacturing equipment.
The increasing usage of solenoids has brought with it the attendant concern of how to identify failed solenoid circuits and more importantly solenoid circuits that are just beginning to degrade. Prior to the rise of computers, solenoids typically provided basic control over equipment, either enabling or disabling the equipment. The power of computers has led to the increasing usage of solenoids to provide fine-tuned control over the performance of equipment. For example, in some automotive transmissions multiple solenoids are used to control complex hydraulic bypass and interconnection paths based upon various engine and drivetrain parameters. As a result of using multiple solenoids, incremental control over all aspects of the transmission is maintained, permitting superior gear shifting, and smoother, more responsive performance. However, using solenoids to provide incremental increases in performance greatly increases the difficulty of identifying which particular devices are failing.
Typically, solenoids are located a significant distance from the computer system that transmits the controlling signals. Signals from the computer are transmitted through wires that are part of a larger wire bundle that is passed through metal enclosures and around tight corners in harsh environmental conditions. Possible failure modes of solenoid circuits include: increased series resistance caused by corroded wires or connections, electrical shorts to chassis caused by insulation breakage, and decreased solenoid inductance. Such a failing circuit can have various effects on equipment performance ranging from intermittent decreases in performance, to continuous performance degradation, and finally to complete failure of the equipment.
When solenoids are used for basic on/off control of equipment, the failure of a device is relatively easy to ascertain since failure of the device normally would result in the equipment not working. However, when solenoids are used to provide incremental increases in performance the failure of a device might result in the equipment still working, although at a lower level of performance. More difficult yet is when a device or the circuit connected to it begins to fail intermittently. The operator perceiving a reduction in performance will take the equipment in for repair. But often, the equipment will not exhibit the reduced performance during the few moments that the repair person inspects it. This starts a cycle wherein the un-repaired equipment is returned to the operator who continues to use the defective equipment. Eventually, the device fails completely, causing the equipment to become inoperative, at which time the repair person is able to diagnose the problem. Unfortunately, sometimes waiting until the solenoid circuit fails completely will lead to the failure of the larger, more costly assembly of which it is a part. As will be appreciated, repairing solenoid circuits by first waiting until the circuit fails continuously has proven to be costly in a number of ways, such as: the operator""s lost time from multiple attempts at repair, the resulting failure of the encompassing assembly, and more lost time from downtime while waiting for the assembly to be repaired.
The difficulty involved in diagnosing and repairing defective solenoid circuits is that many times the circuit will go through a period of reduced performance before complete failure. Conventional methods of diagnosing solenoid circuits entail looking for substantial changes in the electrical waveforms of the circuit. During the period of reduced performance, the electrical waveforms for a solenoid circuit will display subtle differences from normal waveforms, but nothing remotely similar to the substantial changes the repair person is looking for. The exhibited subtle differences are often similar enough to normal waveforms to be indistinguishable. However, waiting for the circuit to degrade to the point that substantial changes in the waveforms occur results in the aforementioned problems with cost, reduced equipment performance, failure of encompassing assemblies, and lost time while attempting to convince a repair person the equipment is defective.
In view of the above, it is desirable to provide a system for detecting changes in the electrical characteristics of a solenoid circuit that is still functional.
It is further desirable to provide a system for determining when a solenoid circuit has completely failed.
Finally, it is desirable to provide a low cost system for detecting changes in the electrical characteristics of a functioning solenoid circuit, which can be implemented in conjunction with a microprocessor.
The present invention offers a method of diagnosing a solenoid circuit to determine if the circuit has begun to degrade. By measuring characteristics associated with the electrical waveform it can be determined if the characteristics of a solenoid circuit are different from the previously measured characteristics of the circuit, or of a known functioning circuit.
A solenoid flyback voltage signal is shaped by a suitable wave shaping circuit for defining the leading edge and trailing edge of the signal. Accordingly, the relative timing of the flyback voltage signal is more important than the peak voltage levels of the signal.
A solenoid driver circuit controls the switching function of the solenoid. When the solenoid is switched off, the solenoid produces the flyback voltage signal. A solenoid control signal from the solenoid driver circuit is output on a first port to a flyback voltage monitoring circuit. The analog flyback voltage signal is processed by a waveshaping circuit and is output on a second port to the flyback voltage monitoring circuit. The waveshaping circuit transforms the analog voltage signal to an approximately square wave logic level signal which is suitable for processing by a logic circuit of the flyback voltage monitoring circuit.
Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood however that the detailed description and specific examples, while indicating preferred embodiments of the invention, are intended for purposes of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.